Excellent considerations, especially the final one about Snape's tragic death. I think that Dumbledore had more than one possible conclusion in mind. The "flash of triumph" in his eyes as Harry is telling him that Voldemort used his blood to be reborn shows that he was fairly certain that Lily's spell was now transfered to Voldemort himself, so that Voldemort's own being alive would keep Harry alive - while Voldemort had no such protection: the one thing he would certainly kill if he tried to kill Harry would be his own Horcrux. But as you point out, there was a second line of defence: if Harry died anyway, and Dumbledore was wrong about Voldemort himself being his defence (or if, for that matter, Narcissa failed to lie to Voldemort - a circumstance not even Dumbledore could have foreseen), then Snape could usurp the Elder Wand and take care of business. it is even possible that Neville amounted to a third possible line of defence, taking his role if both Harry and Snape fell. And if the worst came to the worst, at least all of Voldemort's Horcruxes would have been destroyed, leaving him vulnerable to any angry and vindictive wizard or witch. His sixfold defence against death, that made him an altogether unique danger - since once he won, there would never be a prospect of his ever losing control - would have been removed.

I say, how do you feel about a collaboration along these lines?